A video recorder, like a television receiver, is provided with a so-called ACC (automatic color control) circuit. The ACC circuit regulates the amplitude of the color subcarrier received from the tape to a desired value by evaluating of the amplitude of the color sync signal. On playback, there are two conflicting demands relating to the time constant of the ACC regulation circuit. The comb filter used to eliminate color crosstalk has a finite filtering depth and operates with a delay of one line time with NTSC or two line times with PAL. Prerequisite for a good filtering effect is that the color subcarrier on playback is fed to the comb filter unalienated, i.e. with the phase and amplitude errors caused by the crosstalk. The consequence of this is that the color subcarrier must be deviation controlled (stabilized), during one or two lines, only to such a small extent that no material impairment of the crosstalk attenuation occurs. When the regulation occurs too rapidly horizontally sorted noise appears in the color. This consideration suggests a long time constant for the ACC circuit. On the other hand, the regulation must be so fast that level differences of the video heads, which become relevant after the head change-over as well as level break-downs, caused guiding of the tape, in the head cylinder run-in during the vertical blanking interval and the following picture overscanning are stabilized. If the regulation is too slow then color flicker appears at the upper picture edge. This consideration suggests a short time constant for the ACC circuit.
When selecting the time constant the first consideration is usually deemed the most important. This is so because additional noise would be very disturbing in view of the color quality being reduced by making the color recording. Also, level differences can be kept relatively small by using paired video heads, and level break-downs in the head cylinder run-in are small when the tape is carefully guided. Also, the ACC further reduces level differences which can not be adjusted in the television apparatus. The copying from one video recorder to another is particularly critical as regards color flicker.
It is an object of this invention to avoid the abovenamed disadvantages and to stabilize the color subcarrier amplitude after head change-over and also during picture scanning. With the invention, slower regulation, i.e. a large time constant is chosen or, in the case of an ACC with constant current, a small charging current is used Prior to the start of the picture or just after the head change-over pulse, that is during the period in which the electron beam is visible on the picture screen owing to flyback or overscanning, a switch-over to a quicker regulation, i.e. a smaller time constant or higher charging current is made.
This is possible because of the fact that at the start of a picture during approximately the first 20 lines no visible portion of the screen is scanned and, therefore color flicker and transient phenomena remain invisible during the time the time constant is changed. With the invention after the scanned color subcarrier has gone through the phase shift caused by the head change-over and the amplitude break-down, the color subcarrier is stabilized to its desired amplitude using a short time constant for the adjustment. The amplitude of the color subcarrier therefore will have reached its desired value before the beginning of the visible picture, approximately at line No. 20. The time constant is increased before the first visible line is scanned. Thus, the invention attains an optimum compromise between a large time constant, which is advantageous and necessary for picture reproduction without horizontal noise, and a small time constant, which is advantageous for quick stabilization after head changeover. When applying the invention, the large time constant during the picture can even be increased compared to prior values resulting in advantages with respect to horizontal noise without color flicker appearing at the upper visible picture edge.